The F12-TZ-cCR quartic force field (QFF) methodology, defined here as CCSD(T)-F12b/cc-pCVTZ-F12 with further corrections for relativity, is introduced as a cheaper and even more accurate alternative to more costly composite QFF methods like those containing complete basis set extrapolations within canonical coupled cluster theory. F12-TZ-cCR QFFs produce B 0 and C 0 vibrationally averaged principal rotational constants within 7.5 MHz of gas-phase experimental values for tetraatomic and larger molecules, offering higher accuracy in these constants than the previous composite methods. In addition, F12-TZ-cCR offers an order of magnitude decrease in the computational cost of highly accurate QFF methodologies accompanying this increase in accuracy. An additional order of magnitude in cost reduction is achieved in the F12-DZ-cCR method, while also matching the accuracy of the traditional composite method's B 0 and C 0 constants. Finally, F12-DZ and F12-TZ are benchmarked on the same test set, revealing that both methods can provide anharmonic vibrational frequencies that are comparable in accuracy to all three of the more expensive methodologies, although their rotational constants lag behind. Hence, the present work demonstrates that highly accurate theoretical rovibrational spectral data can be obtained for a fraction of the cost of conventional QFF methodologies, extending the applicability of QFFs to larger molecules.
The low-frequency
vibrational fundamentals of D
2h
inorganic oxides are readily modeled
by heuristic scaling factors
at fractions of the computational cost compared to explicit anharmonic
frequency computations. Oxygen and the other elements in the present
study are abundant in geochemical environments and have the potential
to aggregate into minerals in planet-forming regions or in the remnants
of supernovae. Explicit quartic force field computations at the CCSD(T)-F12b/cc-pVTZ-F12
level of theory generate scaling factors that accurately predict the
anharmonic frequencies with an average error of less than 1.0 cm–1 for both the metal–oxygen stretching frequencies
and the torsion and antisymmetric stretching frequencies. Inclusion
of hydrogen motions is less absolutely accurate but is similarly relatively
predictive. The fundamental vibrational frequencies for the seven
tetra-atomic inorganic oxides examined presently fall below 876 cm–1 and most of the hydrogenated species do as well.
Additionally, ν6 for the SiO dimer is shown to have
an intensity of 562 km mol–1, with each of the other
molecules having one or more frequencies with intensities greater
than 80 km mol–1, again with most in the low-frequency
infrared range. These intensities and the frequencies computed in
the present study should assist in laboratory characterization and
potential interstellar or circumstellar observation.
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